I. Field
The present invention relates generally to data communication, and more specifically to techniques for managing peak-to-average power ratio (PAPR) for multi-carrier modulation in wireless communication systems.
II. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
A wireless communication system may employ multi-carrier modulation for data transmission. Common examples of multi-carrier modulation include orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT). OFDM effectively partitions the overall system bandwidth into a number of orthogonal subbands. Each subband is associated with a respective carrier upon which data may be modulated. The carriers for the subbands may be independently modulated with data, and the modulated carriers are then added together to generate an output waveform.
Multi-carrier modulation has certain desirable characteristics, including the ability to combat multipath effects. However, a major drawback with multi-carrier modulation is high peak-to-average power ratio (PAPR) for the output waveform, i.e., the ratio of the peak power to the average power of the waveform generated by multi-carrier modulation can be high. The high PAPR results from possible in-phase (or coherent) addition of all the carriers when they are independently modulated with data. In fact, it can be shown that the peak power can be up to N times greater than the average power for multi-carrier modulation, where N is the number of carriers.
The high PAPR for the waveform generated by multi-carrier modulation normally requires the power amplifier to be operated at an average power level that is typically much lower than the peak power level (i.e., backed off from peak power). This is because large peaks in the waveform may cause the power amplifier to operate in a highly non-linear region or possibly clip, which would then cause intermodulation distortion and other artifacts that can degrade signal quality. By operating the power amplifier at a back-off from peak power, where the back-off typically ranges from 4 to 7 dB, the power amplifier can handle large peaks in the waveform without generating excessive distortion. However, the back-off represents inefficient operation of the power amplifier during other times when large peaks are not present in the waveform. Thus, it is highly desirable to minimize the PAPR of the waveform so that the power amplifier can be operated closer to the peak power level if desired or necessary.
Various schemes have been introduced to minimize PAPR for multi-carrier modulation. Most of these schemes strive to reduce the PAPR of the waveform itself. For example, one conventional scheme proposes mapping the data to be transmitted into specific codewords that have been specially selected because they are associated with low PAPRs. Another conventional scheme proposes using “peak reduction carriers” that are modulated in a manner to reduce peaks in the waveform. Yet another conventional scheme proposes modulating data on all carriers but with different phases to attempt to reduce the PAPR of the waveform. These various conventional schemes for reducing PAPR may not be applicable for certain multi-carrier communication systems. This may be the case, for example, if the data for all carriers is not available or accessible, as described below.
There is therefore a need in the art for techniques for managing PAPR for multi-carrier modulation in wireless communication systems.